The invention relates to a method for determining the phase angle at a free interface of an optical element provided with a multilayer coating that reflects EUV radiation and/or for determining the thickness of a contamination layer formed on the multilayer coating. The invention also relates to an EUV lithography apparatus, comprising: at least one optical element having a substrate and a multilayer coating that reflects EUV radiation, wherein electrical contact can be made with the optical element for deriving a photocurrent generated during the irradiation with EUV radiation.
At optical elements having a multilayer coating for reflecting EUV radiation, standing waves form during the irradiation. On the basis of the phase angle of such a standing wave at the free interface with respect to the (vacuum) environment, it is possible to draw conclusions about the multilayer coating or about the constitution thereof. Since optical elements that reflect EUV radiation are typically operated in a vacuum environment in a residual gas atmosphere in which the occurrence of contaminating gaseous substances cannot completely be avoided, a contamination layer often forms at the optical elements. On the basis of the phase angle of the standing wave, it is also possible to draw conclusions about the thickness of such a contamination layer that has deposited on the multilayer coating.
The number of charges liberated during the irradiation with EUV radiation as a result of the photoelectric effect at the optical element or the coating varies depending on the phase of the standing wave at the free interface of the optical element. On the basis of the intensity of the photoelectron current (hereinafter for short: photocurrent), it is thus possible to draw conclusions about the phase angle at the free interface and/or about the thickness of the contamination layer.
WO 2005/091076 A2 has disclosed a method for determining the thickness of a capping layer system on a multilayer coating that reflects EUV radiation. Determining the thickness of the capping layer system involves firstly determining a first phase shift of a standing electromagnetic wave at the free interface of the multilayer coating. For this purpose, both the reflectivity and the photocurrent are measured in a wavelength-resolved manner during the irradiation of the free interface with EUV radiation and the first phase shift is determined on the basis of the profile of the photocurrent in the range of maximum reflectivity. A comparison with an already known profile of the photocurrent in the case of a second phase shift of the corresponding standing wave is also performed in order to determine a thickness difference or the thickness of the capping layer system on the basis of the difference between the first and second phase shifts. Instead of a wavelength-resolved measurement of the reflectivity and of the photocurrent, an angle-resolved measurement can also be effected. The measurement of the reflectivity and of the photocurrent can, if appropriate, also be effected for specific combinations of wavelengths and angles of the incident EUV radiation.
In WO2005/091076 A2, photoelectrons ejected from the multilayer coating by the EUV radiation are trapped by an electron trapping device, which can be designed e.g. as a vacuum wall, plate or wire grid. The current generated by the trapped electrons is detected by an ammeter. A ring, a cylinder or a grid can be used for generating a defined electric field between the multilayer coating and the electron trapping device.
An EUV lithography apparatus provided with a detection device for photoelectrons has also been disclosed by DE 102 09 493 B4, wherein use is made of an electron trap for trapping the electrons, which electron trap can be designed e.g. as a detection grid or as a detection ring. The current generated by the trapped electrons is detected by an ammeter. The gas composition of the residual gas atmosphere of the EUV lithography apparatus is regulated on the basis of the measured photocurrent.
EP 0 987 601 A2 describes an exposure apparatus in which a detection device is provided adjacent to a reflective surface in order to detect electrons generated during the irradiation of the reflective surface with X-ray radiation as a result of the photoelectric effect. On the basis of the quantity of photoelectrically generated electrons, the radiation dose occurring during the irradiation with EUV radiation at the mask can be determined and the radiation power of the X-ray radiation can be limited. The detection device can have a grounded ammeter or a voltmeter. The ammeter can be connected to a reflective multilayer coating of a mirror of the exposure apparatus via an electrical cable.